1. Field of the Invention
The present invention relates to a coating compositon for a coated electrode for arc welding, and a coated electrode covered with the composition.
2. Description of the Prior Art
Conventionally coating compositions containing a considerable amount of starch and dextrin, for example, a high-cellulose coating composition, have been widely used for coating electrodes for arc welding. These coated electrodes, however, have been confronted with defects that when the welding is done with a high current density (220A for a vertical downward welding with an electrode of 4.0 mm diameter) the coating on the electrode is excessively heated due to the Joule effect caused in the core wire so that organic compounds contained in the coating composition denatures or decomposes to cause a remarkable phenomenon of electrode burning, resulting in occurrence of pits and blow holes in the deposited metal, and failing to exert various functions expected from the coating composition. Further, the adhesion force of the coating is weakened so that cracking occurs more often in the cup of coating formed at the tip of the electrode during the welding and the coating easily chips off to cause unstability of the arc and short-circuiting of the electrode, resulting in remarkable difficulty in the welding operation.
Meanwhile, in recent years various high-tensile strength steel pipes having a tensile strength more than 50 kg/mm.sup.2 and thick-walled mild steel pipes have been increasingly used for building oil and gas pipe lines in cold regions, and joints of these steel pipes are required to have enough strength and toughness to meet severe service conditions in these applications. Conventional electrodes have been found unsuccessful in providing such weld joints.